In present commercial practice, when dense soils or bedrock are at moderate depths, steel "H" section piles, precast piles, or pipe piles may be driven to these soils or rock. In this way, static load capacities in excess of 80 tons are achieved. Since these piles derive their principal support at their tip, they are best categorized as "end bearing piles."
However, there are numerous conditions in which pile design permits piles to be successfully driven into granular or cohesive soils, or mixtures thereof, for the supporting of those piles. In such situations, the piles distribute their load by a combination of friction forces acting along the side of the pile and by "end bearing" forces acting beneath the tip of the pile. These piles may be steel "H" piles, precast piles, pipe piles, or mandrel-driven shell piles and are best denominated "friction piles."
Frequently a soil profile is encountered wherein unsuitable soils, i.e., those which will compress or consolidate excessively, are underlain by bearing soils which are of only moderate or low density. While conventional "friction piles" might ultimately achieve adequate penetration for support in such soil, the depth of penetration required may be such that it can be achieved much more economically with the new and improved piles of the present invention. In accordance with the invention, a less expensive pile is provided which can be driven with greater facility through intervening layers, if any, of semi-suitable soils to the ultimate bearing layer without recourse to the time-consuming and costly special methods which heretofore might be required for state of the art types of piles.
As has been described by G. G. Meyerhof in a thesis entitled "The Ultimate Bearing Capacity of Wedge-Shaped Foundations," the ultimate bearing capacity of a foundation may be markedly increased by using a wedge of shallow depth and making the wedge of a rough surface, e.g. concrete. Numerous prior art pile devices have made use of this fundamental general "wedge" principle. The Raymond "Standard Pile," which is heavily tapered, embodies this principle. Monotube piles utilize a variety of taper configurations in conformity with this principle. Likewise, Franki piles, which are in a sense an "in situ" spread footing, nevertheless embody this wedge theory. Indeed, this general concept has also been utilized by positioning a structural, wedge-shaped mass, of larger area than the pile, at the tip (very bottom) of the pile as described in Merjan Pat. No. 3,751,931.
The present invention is directed to certain pile structures in which substantial advantages are derived from positioning a new wedge-forming structure spaced from the tip of the pile.
The principle upon which the present invention is predicated is that the frictional value betwen soil and soil is higher than that between steel and soil or concrete and soil. Accordingly, a soil-soil interface results in the highest value of support. This concept has been applied in a different fashion in prior art piles. For example, the concept is demonstrated by the increased capacity of corrugated shell piles versus smooth-sided pipe piles of the same length and diameter. The higher capacity of the shell results from the fact that the soil is locked in the valleys of the shell corrugations. Hence, the mode of failure is a function of the sheer strength of the soil rather than the friction between steel and soil, the former being a much higher value. Another example is found in steel "H" piles. It is well known that soil "locks" between the flanges and against the web of such piles. The locked soil results in a mode of support based on the frictional value between soil and soil.
The new and improved pile of the present invention employs a "wedge-forming element" or "wedge-former" which is spaced upwardly on the pile shaft from the tip rather than forming an actual wedge at the very tip. As a result of this unique positioning of a new "wedge-former" on the pile, during driving of the pile soil is forced to form in situ a soil wedge and to interface with other soil as the pile is driven to its ultimate depth. Specifically, soil collects under the shoulder and the tapered outer wall of the "wedge-former" and is, in effect, "locked" into the "wedge-former" (between the "wedge-former" and the shaft of the pile). The locked soil functions as a true wedge against other soil and it is this soil-soil effect which makes the new pile particularly advantageous, since, as discussed above, the frictional value between soil and soil is higher than that between steel and soil or concrete and soil. For this reason, the "soil wedge" formed in situ about the new pile provides a higher value of support at shallower driven depths than that found in earlier piles.
Because the soil wedge formed in accordance with the principles of the invention enables the pile to mobilize the soil's resistance more efficiently than known piles, the new pile need not be so massive as the Franki-type pile or the Merjan-type precast wedge tips. Hence, it lends itself to installation more economically (with conventional pile driving equipment and often to lesser depths) in a large range of soil conditions. Furthermore, the wedge-forming piles of the present invention are superior to the massive, precast concrete type of piles with integral wedges in that the new piles may more readily be driven through layers of semisuitable soils to the ultimate bearing layer without recourse to time-consuming and costly methods such as jetting or predrilling. Also, the new and improved piles of the invention avoid the uncertainties inherent in the formation of Franki-type piles, each of which is constructed according to a variety of guidelines furnished for implementation at the site. The new pile may be driven with impact pile hammers having energies in the range of approximately 15,000 to 36,000 ft.-lbs, per blow and the size of the retaining device may be readily varied to suit soil/load/hammer interrelationships.
The piles of the present invention may take any of several preferred forms. For instance, the "wedge-former" may be in the form of a truncated cone of precast concrete inserted onto a pipe pile and attached at an appropriate place thereon. Alternatively, the shaft and wedge-former may be integrated in the form of a unitary concrete pile. Or, in a particularly advantageous embodiment, a precast concrete point which includes an appropriately positioned wedge-former and a short section of pipe cast into its upper portion may be employed. In using this embodiment, the point is first planted into the ground. Then, the remainder of the pipe pile is connected to the section of pipe which was cast in the wedge and the pile is driven into the soil in the usual way.
It should be noted that the invention is not limited to the use of pipe piles; as will be apparent to those skilled in the art, other types of piles may be utilized. Similarly, the shape of the wedge may be varied somewhat for adaptation to different situations.
It is an object of this invention to provide a pile with improved penetration and superior support characteristics.
It is a further object of the invention to provide a pile which may be easily and economically driven into soil of inferior support quality.
For a more complete understanding of the above and other features and advantages of the invention, reference should be made to the following detailed description of preferred embodiments thereof and to the accompanying drawings.